1. Field of the Invention
The present invention relates to a gas sensor and manufacturing method thereof. More particularly, the present invention relates to a gas sensor using carbon nanotubes and metal oxide as a sensing film.
2. Description of the Related Arts
Naturally occurring gases such as carbon monoxide (CO), nitrogen oxides (NOx), hydrogen sulfide (H2S), or methane (CH4) can be hazardous to humans. These toxic gases are colorless and tasteless and cannot be easily detected by human senses. When the concentration of toxic gases in an environment exceeds an allowable range, symptoms of headache, dizziness, sickness, or even shock or death can result. Gas analyzers provide real-time monitoring of gas content in airtight or unventilated environments, providing timely notification when the concentration of toxic gases exceeds a threshold.
Atomic/molecular absorption spectrometry, atomic/molecular fluorescence spectrometry, and gas chromatography are commonly used in chemical laboratory and quality control for gas analysis. These gas analysis instruments have the advantages of high accuracy, high sensitivity, and low detection limit; however, their application is limited due to a large profile with low portability, high power consumption, structural complexity, and high cost. It is, therefore, important to provide a simply equipped gas sensor.
Gas sensors detect gas concentration and convert the concentration into an electric signal. Traditional gas sensors include, for example, electrochemical gas sensors and metal oxide semiconductor gas sensors.
Electrochemical gas sensors detect gases by measuring an electric current or pressure produced by an oxidation reduction reaction resulting from the dissolution of the target gas in a liquid electrolyte between two electrodes of an electrochemical tank. These gas sensors are applicable at room temperature; however, they have a short lifetime due to the corrosive property of the liquid electrolyte. In addition, calibration of the sensor baseline is required since chemical buildup at the reference electrode may cause baseline drift. Moreover, electrochemical gas sensors are usually costly.
Metal oxide semiconductor (MOS) gas sensors utilize resistance changes depending on gas contents absorbed by metal oxide to detect gas concentration changes. They can be simply prepared or combined with microelectromechanical systems (MEMS) to be portable. With these properties, MOS gas sensors are highly useful. A MOS gas sensor can include a ceramic substrate, a pair of measuring electrodes, a sensing layer, and a heater. The sensing layer is usually a polycrystal and porous layer of metal oxides such as SnO2, ZnO, Fe2O3, In2O3, WO3 and the like. Examples of SnO2 base MOS gas sensor includes U.S. Pat. Nos. 4,535,315, 5,185,130, 5,273,779, 5,427,740, 5,624,640.
U.S. Pat. No. 4,535,315 discloses a process for the manufacture of a sensor including preparing a fine tin oxide powder using stannic chloride, preparing a paste by dissolving the tine oxide powder into ethylene glycol, applying the paste on a ceramic substrate, and baking the substrate to form a SnO2 sensing layer. The SnO2 gas sensor selectively detects alkane gases such as methane gas, propane gas and butane gas. The detection of alkane gas is based on that the absorbed alkane gas decreases the electrical resistance of the sensor. However, heat desorption of the absorbed gases at 200 to 500° C. is required, and the sensitivity of the sensor is not satisfactory.
In order to enhance the sensitivity of SnO2 gas sensor, addition of other elements to the SnO2 layer was proposed by U.S. Pat. No. 5,185,130. The proposed sensor selectively detects nitrogen oxide and hydrogen gas in an atmosphere containing carbon monoxide, methane, nitrogen oxide, and hydrogen gas. The preparation of the sensor includes depositing a thin film of tin and bismuth on a ceramic substrate by vacuum evaporation followed by thermally treating the film deposited on the substrate. The thermal treatment includes the following cycle: increasing the temperature from ambient to 300-350° C. over 5-35 minutes and maintaining the temperature for 1-3 hours; increasing the temperature to 400-450° C. over 2.5-3.5 hours and maintaining the temperature for 2.5-3.5 hours; increasing the temperature to 470-500° C. over 4-6 hours and maintaining the temperature over 3-4 hours. The prepared sensor contains Bi2O3 of 5-7% by atomic weight. High throughput production cannot be easily achieved due to the requirement of vacuum equipment; the thermal treatment also increases the cost; in addition, the working temperature of the sensor is 200-500° C.
Another example of the enhancement of sensitivity was proposed by U.S. Pat. No. 5,273,779. The addition of noble metals to the SnO2 substrate enhances the sensitivity of the sensor via the catalyst effect. The disclosed SnO2 gas sensor for the detection of alcohol and H2 is prepared by immersing an Al2O3 substrate into an organic tin solution, forming a buffer layer on the substrate by spin coating, sintering the buffer layer at 800° C. over 5 hours, immersing the substrate into an organo-metallic solution of Au and Sn, forming a SnO2 gas sensing layer by spin-coating and heat-treating at 800° C. over 5 hours, screen-printing electrodes with heat treatment at 600° C., immersing the substrate into a organo-metallic solution of Au and Pd, forming a catalyst layer on the SnO2 layer with heat treatment at 600° C., and coating a second gas sensing layer (with a organo-metallic solution of La and Sn) on the catalyst layer with heat treatment at 600° C. over for 5 hours. The process is complicated and costly due to noble metals and multiple heat treatments. In addition, the gas sensor cannot be used at room temperature.
U.S. Pat. No. 5,427,740 discloses a SnO2 gas sensor for the detection of H2, CO, and CH4. The preparation of this gas sensor is similar to U.S. Pat. No. 4,535,315, except for that the slurry including Sb is heat treated at 1100-1600° C. prior to coating on the substrate. A tin oxide gas sensing layer containing Sb2O3 is formed after 800-1000° C. heat treatment. This preparation is costly due to the heat treatment, and the gas sensor also needs to be used at 280-400° C.
U.S. Pat. No. 5,624,640 discloses a sensitivity-enhanced gas sensor for the detection of NO, NO2, and N2O4 with a SnO2 gas sensing layer containing Ta, Nb, Sb, or W. The SnO2 layer is covered by a gas convert layer composed of TiO2, ZrO2, SiO2, or Al2O3 and a catalyst such as Pt. Nitrogen oxide is detected by the gas convert layer and oxidized to NO2 or N2O4, and NO2 or N2O4 is then detected by the SnO2 layer. This gas sensor is costly and has to be used at 180-400° C.
The drawbacks of these gas sensors include low sensitivity, selectivity, and stability with gas. To accelerate desorption of gas absorbed on the sensing layer of metal oxide, MOS gas sensors are usually applied at higher temperatures such as 300 to 450° C. to enhance recovery time. Longtime operation at high temperatures may, however, causes irreversible changes in the electrical properties of metal oxides, leading to signal drift. The heating requirement of these gas sensors also evokes several problems such as increased size of the gas sensor, power consumption, and temperature control. Therefore, the cost of the gas sensor cannot be reduced.
Nano-materials have advantage properties associated with their special surface and volume effects and can be used in porous sintered films to increase reaction surface and sensitivity. U.S. Pat. No. 6,059,937 (2000) discloses a SnO2 gas sensor prepared by depositing a SnO2 nano-level film on a ceramic substrate by ion beam sputtering, and depositing Pt or Pd electrodes on the substrate. The sensor detects CH4 and C3H8 at a lower temperature, such as 150° C. for CH4 and 190° C. for C3H8; however, ion beam sputtering and Pt or Pd electrodes are costly, and the working temperature cannot be reduced to room temperature. U.S. Pat. No. 6,134,946 (2000) discloses a SnO2 gas sensor for the detection of carbon monoxide, hydrocarbons, and organic vapors. The preparation comprises depositing tin oxide sol on Pt electrodes of a sensor. The thin film of tin oxide has a nano-crystalline structure with good stability. However, the sensor still cannot be used at room temperature.
The above mentioned sensors have a common problem—unable to be used at room temperature; therefore, researchers are focusing on the study of a sensor with low working temperature. An example is U.S. Pat. No. 5,448,906 which discloses a gas sensor with a working temperature of room temperature. A light source such as UV or LED can be equipped on the gas sensor. When sensing gases, the light induces gas desorption from the sensing film. The disadvantage of the sensor is the difficulty of controlling sensor temperature since the temperature of the sensor is increased by lightening time. In addition, the design for light induction is complicated and the cost cannot be reduced. There is, therefore, still a need for a gas sensor with low cost and workable at room temperature.